Two separate things.
1. Your W-4 is used to estimate your correct withholding. It has no bearing on how you file your return. You file your tax return with your correct facts no matter what you put on the W-4. If you had too much tax withheld, you will get that back as a refund, and if you had too little tax withheld, you will owe a payment. Changing your W-4 will not get you any money back from last year, you have to file your tax return.
2. Going forward, you can change your W-4 as often as you like, so your withholding is accurate for your changing tax situations. The IRS withholding estimator may be useful, especially if you have more than one job, a working spouse, or lots of kids or deductions.
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